purgedata: remove /etc/ceph too

Signed-off-by: Sage Weil <sage@inktank.com>
install: do not support argonaut installation

This is untested and unsupported.  If users really want it, they can use
the --dev=argonaut approach (which is likely what they want anyway!).

Signed-off-by: Sage Weil <sage@inktank.com>